ADDEE3S OF PROF. A. M. MAYER ^ 479 



currents, contained in planes perpendicular to the rectilinear current. 

 The result of the calculation, confirmed by experiment, was that the 

 planes of the circular currents would, supposing them movable, 

 arrange themselves parallel to the rectilinear current. If like trans- 

 verse currents pass over the whole length of a magnetic needle, 

 the cross direction which, in the experiment of Oersted, seemed an 

 inexplicable anomaly, would become a natural and necessary fact. 

 Is it not then evident to all how memorable would that discovery 

 be that would rigorously establish the fact that to magnetize a 

 needle is to excite, to put in motion around each molecule of the 

 steel, a small circular, electrical vortex? Ampere fully realized the 

 wide reach of the ingenious gene'ralization that had taken possession 

 of his mind ; and he hastened to submit it to experimental proofs 

 and numerical verifications, which, in our day, are the only processes 

 considered entirely demonstrative." 



About this time Arago found that the conjunctive wire of the 

 battery had the property of causing iron filings to arrange them- 

 selves around it in concentric rings. Guided by Ampere's discovery 

 that a helix conducting a voltaic current had properties similar to 

 those of a magnet, Arago inferred that these properties could be 

 given to iron and steel by placing wires or bars of these substances 

 in the interior of one of Amp^re^s helices. Experiment showed that 

 his inference was correct. The same effects he obtained by passing 

 electrical discharges from an ordinary frictional electrical machine or 

 from a Leyden jar through a helix inclosing a steel needle. 



In subsequent memoirs, exhibiting great philosophic acumen and 

 marked ability in the application of mathematical analysis to the 

 elucidation of physical phenomena. Ampere developed the conse- 

 quences of the general laws he had previously discovered. 



In 1821, six years before Henry began his work, Faraday — then 

 30 years of age, and as yet an assistant of Davy — published his first 

 paper on electrical research. In this he shows that a wire conveying 

 an electrical current can be made to rotate around the pole of a 

 magnet. He then reverses the action, and holding the wire at rest 

 makes the magnetic pole rotate around the wire. These phenomena 

 were shown by Ampere to be entirely conformable to his hypothesis 

 of the electro-dynamic nature of a magnet. • • ■•; ' -ii '•• ■ 



